The invention relates to a propfan or propeller engine having a shroud which surrounds the propfan or propeller and, more particularly, to a shroud having a variable geometry. The direction of the flow inside the shroud duct is reversible for the reversal of the thrust by adjustable fan blades or propeller blades.
In particular, the invention relates to so-called "UHB-aircraft engines" (Ultra-High-Bypass), thus, the invention relates to engines with a very high bypass ratio, for example, on the order of 10:1. A typical engine of this category has two shrouded contra-rotating propfan rotors. For maintaining an optimal operating point during the different phases of the flight, the blades of the propfan rotors are constructed to be adjustable in their pitch. Furthermore, by a corresponding rotor blade adjustment, a surging of the compressor is avoided.
In order to keep the landing ground roll of an aircraft as short as possible, today's aircraft engines have a thrust reverser. For this purpose, German Patent Document DE-OS 24 46 548 discloses moving the blades of a fan or of a propfan in the reverse position. In this case, the air for generating a reverse thrust is taken in around the trailing edge of the shroud and is blown out inside the shroud duct toward the front in the flight direction.
The quality of the flow around the trailing edge of the shroud is of decisive significance for the braking effect during thrust reversal. In the case of the trailing edge of the shroud which, for a better flow-off in the normal flying operation, is usually designed to be sharp-edged, the flow around the trailing edge in the case of the reversal of the flow direction results in burbling and thus in considerable pressure losses. The available maximal reverse thrust is therefore reduced in an unacceptable manner.
In the case of the engine that has become known in the above-mentioned patent document, during a thrust reversing operation, the flaps which are pivotally connected to the outlet of the shroud are swivelled out in such a manner that they form a bell-shaped inlet opening. The resulting enlarged inlet cross-section has the purpose of causing an improved flow through the fan and thus a larger reverse thrust. However, a burbling in the case of the flow around the sharp trailing edge of the flap with the connected impairment of the thrust reversing effect cannot be prevented or limited. The turbulent flow against the rotor blades caused by the burbling may result in a dangerous excitation of vibrations of the rotor blades.
Based on the above, there is therefore needed an engine of this type developed in such a manner that the aerodynamic flow around the obtuse rearward shroud end and the quality of the flow in the shroud duct are improved and thus the danger of an excitation of vibrations of the rotor blades is reduced. In addition, a sufficient air supply to the core engine must be ensured at the same time.
In an advantageous further development, the displaceable nozzle ring, for the thrust reversal position, is to be moved axially into a ring-shaped pocket of the shroud. This embodiment is distinguished by its simple and light construction. The servicing expenditures remain low because the nozzle ring is formed of one component. This ensures considerable reliability.
In an alternative embodiment, the nozzle ring is composed of several ring segments. It is therefore possible to displace the ring segments with respect to the stationary shroud by swivelling levers axially forward and radially so far toward the outside that the trailing edges of the ring segments form, with the trailing edge of the stationary shroud, an edge of greater radial cross section which promotes the surrounding flow. In this case, the higher air resistance of the shroud when the ring segments are moved out proves to be advantageous without any significant increase of the maximal diameter which would reduce the ground clearance in the case of wing-mounted engines.
Another thrust reversal and braking effect is achieved by an alternative embodiment in which the rearward end of the shroud is constructed as a movable nozzle ring divided into separate ring segments. The individual ring segments, by means of their swivelling levers, can be moved axially forward and radially toward the outside as well as while turning about their body axis which is situated perpendicularly with respect to the translatory moving plane. The ring segments can be moved into such a position that, in their moved-out end position, they assume a position which is offset from the rearward end of the stationary shroud and adjusted with respect to the outer air flow. The trailing edge of the respective ring segment points to the core housing.
As a result, in the moved-out condition, the interior side of the ring segments forms flow ducts with the exterior side of the shroud. The flow ducts guide the air flow in an aerodynamically advantageous manner around the rearward end of the shroud.
In a further development of the invention, swivelling flaps are arranged in sections on the circumference of the shroud, which flaps, in the moved-out condition not only promote the flow around the rearward shroud end but also the face of the engine and thus increase the braking effect. The cause of the favorable surrounding flow is the separating whirl which is generated on the rearward edge of the flap and which has a rounding-off effect for the rearward shroud end. The surrounding flow therefore takes place in a larger radius in which case the risk of burbling is reduced.
In another expedient development, the flaps are coupled with the nozzle ring in such a manner that their movement takes place simultaneously. As a result, an optimal flow-around quality is ensured.
In order not to negatively influence the resistance in normal flight, the outer contour of the moved-in flaps closes off flush with the outer contour of the shroud.
In a preferred development, the shroud duct, which is formed between the core housing, on the one hand, and the interior side of the shroud with the nozzle ring, on the other hand, extends in a divergent manner, that is, forward, in the flow direction during the thrust reversal. When the nozzle ring is moved in, the inlet cross-section will increase and the flow losses will therefore be reduced. The flow losses are a function of the expansion ratio of the shroud duct and of the flow Mach number at the inlet. In a further development, an effective function of the thrust reverser is indicated while the blades are adjusted at the same time as the nozzle ring is displaced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.